An inverter device such as a three-phase voltage-source inverter includes single-phase bridges constituting three phases. Each single phase includes a pair of series-connected switching elements (one is referred to as a positive switching element and the other as a negative switching element). The switching elements of each bridge are on-off controlled by PWM control so that DC power is converted to AC power. In this case, a switching pattern is determined so that dead time is interposed between OFF periods of the paired switching elements in order that both switching elements in each bridge may be prevented from being simultaneously turned on. Accordingly, the DC power source can be prevented from being short-circuited via each bridge.
However, another short-circuit current needs to be prevented as well as the above-described short-circuit current. More specifically, short-circuit current passing through a freewheeling diode needs to be overcome. Each pair of switching elements constituting an inverter is generally provided with a freewheeling diode connected in reverse parallel with each pair. For example, consider the case where the positive switching element is turned on under the condition that both positive and negative switching elements are turned off during dead time. It is known that in this case, current having passed through the switching element turned on becomes short-circuit current passing through the freewheeling diode of the negative switching element in the reverse direction. It is also known that the short-circuit current results from recovery current due to residual carriers remaining in the freewheeling diode.
The recovery current has a sharp needle-shaped waveform to result in large surge voltage which induces intense noise, fluctuates a body chassis potential of a vehicle, enlarges control errors and increases switching loss. The recovery current thus results in various failures. In view of these failures, a technique has conventionally been proposed to reduce recovery current in a three-phase voltage-source inverter. In the proposed technique, current-limiting reactors are interposed in series to the switching elements respectively. Freewheeling diodes are connected in parallel to the series circuits of the current-limiting reactors and the switching elements respectively. The proposed configuration requires no treatment of switching control software. However, steady current normally passes through the current-limiting reactors in the proposed configuration. This results in disadvantage in inverter's use application in which loss should be avoided.